Longitudinally adjustable gas springs of this type are for example known from JP-U-51-153 and EP-B1-0 219 362. In this case a conical or frusto-conical transition section is formed between the two cylindrical sections of the valve pin. For actuation of the valve the valve pin is pushed into the valve body in the direction towards the adjacent housing chamber to such an extent that the annular seal neighbouring the housing chamber is completely lifted off the cylindrical section of greater diameter, so that a passage is realized from the inner chamber of the valve body to the adjacent housing chamber. The disadvantage of this solution resides in that the annular seal neighbouring the housing chamber is heavily milled and sheared upon closing of the valve, when the cylindrical section of greater diameter of the valve pin is pulled through the annular seal and widens it, as a result of which there is the risk of the annular seal to be damaged, which may render the gas spring unserviceable. Comparable problems arise, when the annular seal sealing outwards is bridged by the valve pin being pushed very deeply into the valve body for the purpose of filling compressed gas into the gas spring.